1. Field of the Invention
This invention relates to a method of improving the diffusion barrier properties of the gate oxide of MOSFET transistors.
2. Brief Description of the Prior Art
The continued reduction in the geometry of MOSFET transistors has resulted in the requirement of continually shorter gate lengths (&lt;0.3 .mu.m). This geometry reduction has also required a reduction in gate oxide thickness to about 5 nanometers and reduction in operating supply voltage in order to support the minimum gate length without excessively high threshold voltages. Since n-type and p-type polycrystalline silicon (polysilicon) is commonly used as the gate electrode material in MOSFETs and with high gate electrode doping being essential to minimize depletion layer depth in the gate electrode, dopant diffusion from the polysilicon through the gate oxide and into the channel region of the MOSFET becomes an increasing problem with the reduced gate oxide thickness. This problem is particularly critical for the case of a boron-doped polysilicon gate in a p-channel MOSFET having a p-type polysilicon gate electrode.
Attempts to minimize this problem in the fabrication of state-of-the-art MOSFET transistors have generally involved improvement in the diffusion barrier properties of the gate oxide. Prior art methods of providing improved diffusion barrier properties to the gate oxide of MOS transistors have involved the introduction of nitrogen into the ambient during processing when the surface of the gate oxide is exposed in the form of N.sub.2 and/or nitrous oxide (N.sub.2 O). While providing desirable diffusion barrier properties to the gate oxide by forming an oxynitride at the exposed surfaces of the gate oxide, this method also results in reduction of surface carrier mobility since the nitrogen is present at the channel/gate oxide (Si/SiO.sub.2) interface.
Attempts have also been made to improve the barrier properties of SiO.sub.2 using re-oxidized, nitrided oxide (ROXNOX) and oxides grown using nitrous oxide ambient. In both cases, the heavily nitrided region is near the Si/SiO.sub.2 channel interface and degrades carrier mobility. Other methods of incorporating an oxy-nitride region in the gate oxide away from the channel interface, such as oxynitride deposition or conventional ion implantation suffer from the problem that the oxides are so thin, .about.10 nm or less, that they require very precise deposition control or very low ion implantation energies. It is therefore apparent that improved procedures are required for the new generations of MOSFETs for formation of the diffusion barrier.